With the recent demand for miniaturization, low power consumption, and the like of electronic devices, power supplies incorporating a switching circuit that repeats turning on and off periodically, which are so-called switching power supplies, have been more frequently used. The use of PWM (Pulse Width Modulation) control for adjusting a duty ratio of an input pulse signal, for example, in each switching power supply enables adjustment of an output voltage according to an operating state of an electronic device.
In addition, there is an increasing demand for a further reduction in power consumption by improving the conversion efficiency. In the International Energy Star Program (Energy Star), for example, a power factor correction (PFC) is necessary for electronic devices of 75 W or higher. The PFC control includes a single mode and an interleave mode. In the interleave mode, switching operations are complementarily performed by two PFC control pulse signals having a phase difference π (180°). As a result, two coil currents having a phase difference π are generated. Accordingly, in the interleave mode, electric power twice as large as that in the single mode is obtained. Further, the ripple of an output current is small, which leads to a reduction in the capacity of a capacitor for reducing the ripple.
Patent Literature 1 discloses a switching power supply which employs a critical mode interleaved PFC control. Patent Literature 1 discloses a technique for detecting only a zero current in one coil current and automatically generating PFC control pulse signals having a phase difference n, and a technique for detecting a zero current in each of two coil currents and generating two PFC control pulse signals. In the latter case, the critical mode for both coil currents can be ensured, which results in improvement in efficiency.
Note that Patent Literature 2 discloses a watchdog timer which detects an input of a clear signal within a certain period of time to be counted, in accordance with execution of a program, and outputs a reset signal to a computer to inform an abnormality. This watchdog timer includes signal control means that allows the clear signal to be input only before a set time with respect to an end time of the certain period of time.